Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. One method for determining a heart rate of a person involves using a photoplethysmographic (PPG) sensor. Such a sensor typically includes one or more light sources and one or more detector. During use, the one or more lights sources may illuminate a portion of a person's skin. Blood flowing through vessels within the illuminated portion of the skin reflects a portion of the emitted light, which the one or more detectors may detect over a non-zero time period, thereby providing a PPG signal. A pulse or heart rate can thus be determined from the PPG signal, as changes in the intensity of the detected light that correlate to changes in blood flow through the illuminated area resulting from the person's heart pumping blood. However, a number of factors may affect the intensity of the light detected by the one or more detectors. For instance, a person's breathing may cause respiratory-induced variations in a PPG signal. Assuming a regular respiration rate over the non-zero time period in which a given PPG signal is captured, it may be possible to estimate a respiration rate from one or more respiratory-induced variations in the PPG signal.